Conventional logic circuit devices generally operate with binary logic, such that each circuit element operates only with two states. When large amounts of data are to be processed, a very large degree of integration is required in electronic circuits in order to enable this to occur. Multiple-state logic devices have been proposed i.e. devices with more than two stable states with a view to reducing the total size of the devices and to accelerate information processing. For example, multiple-state resonant-tunnelling bipolar transistors (RTBTs) have previously been proposed--see F. Capasso, S. Sen and F. Beltram, in High-Speed Semiconductor Devices ed. S. M. Sze (John Wiley & Sons, N.Y. 1990) Chap. 8 "Quantum-Effect Devices": However, these prior devices suffer from the disadvantage that RTBTs need to be stacked in order to achieve a high degree of multiplicity in logic operation and as a result, instability can occur due to background conductance, which can limit the multiplicity that can be achieved.